Understanding the Gas Sensing Mechanism for NO2 on Unloaded and K-Loaded ZnO

Abstract

Unloaded and K-loaded ZnO gas sensors were prepared via screen printing. After material characterizations by means of scanning electron microscopy, X-ray photoelectron spectroscopy and X-ray diffraction, the sensors were investigated by DC resistance measurements combined with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). At an operating temperature of 250 °C, the sensors were exposed to 0.8–1.9 ppm NO2 in 10 % RH @ 25 °C. In the first part, two unloaded samples with different morphologies were investigated. During the reaction between ZnO and NO2, the formation of differently coordinated nitrites and nitrates and/or Zn-O bonds was observed, all of which led to an increase in resistance. The adsorption of a comparatively increased amount of hydroxyl groups during NO2 exposure together with a higher concentration of oxygen vacancies explains the preferential formation of Zn-O bonds in one of the two samples. For the second part, K was ultrasonically dispersed on ZnO powder. In contrast to unloaded ZnO, K-loaded ZnO responds with a decreasing resistance to NO2 and recovers extremely slowly, indicating irreversible surface reactions. The DRIFTS results show that especially free nitrate was formed, suggesting the formation of KNO3. During this process, the desorption of a surface oxygen atom occurs, which explains the decreasing resistance. When all K reacted to KNO3, NO2 is adsorbed as nitrite species and the resistance increases again.